1. Field of the Invention
The present invention relates generally to an apparatus and method to transmit/receive signals in a mobile communication system. More particularly, the present invention relates to an apparatus and method to transmit/receive signals according to a coding rate in a communication system.
2. Description of the Related Art
Next generation communication systems are evolving to provide mobile stations (MSs) with services capable of high-speed, high-capacity data transmission/reception. In the next generation communication systems, various transmission/reception schemes, such as Hybrid Automatic Repeat reQuest (HARQ) scheme and Adaptive Modulation and Coding (AMC) scheme, have been proposed to support the high-speed, high-capacity data transmission/reception. To use the HARQ and AMC schemes, the next generation communication systems should support various coding rates.
It is known that the next generation communication systems are high in performance gain when they perform high-speed data transmission/reception with turbo codes. The systems positively consider using Low Density Parity Check (LDPC) codes that can increase reliability of data transmission by effectively correcting errors caused by noises occurring in transmission channels. However, use of the LDPC code has shortcomings in terms of the coding rate. That is, the LDPC code is not free in terms of the coding rate because its generated codeword has a higher coding rate due to a characteristic thereof. Most of the currently proposed LDPC codes have a coding rate of ½, and only some of them have a coding rate of ⅓. Because the LDPC codes have limitations in terms of the coding rate in this way, it is inappropriate to use the LDPC codes for the schemes that should support various coding rates.
To realize a lower coding rate, it may be possible to find a degree distribution showing the best performance with use of a density evolution technique. However, an LDPC code having the degree distribution showing the best performance is hard to realize due to several restrictions such as a cycle structure in a bipartite graph and its hardware implementation.
Because the LDPC code has limitations in terms of the coding rate due to its characteristic as described above, there is a need for a signal transmission/reception scheme supporting various coding rates from a low coding rate to a high coding rate in a communication system using the LDPC code.